In an article by J. E. Goell entitled "An Optical Repeater With High-Impedance Input Amplifier," published in the April 1974 issue of the Bell System Technical Journal, pp. 629-643, the block diagram of a repeater for use in a digital optical communication system is described. Typically, such a repeater includes a linear optical detector followed by suitable amplifiers, equalizers and filters, and a regenerator which includes a level discriminator for distinguishing between the two binary states of the signal. The regenerated pulses are then amplified, and the amplified pulses used to modulate a suitable light source.
More recently, Copeland et al, in an article entitled "Optically-Switched PNPN Light-Emitting Diodes" published in the 1977 IEEE International Electron Device Conference Technical Digest, pp. 580-581A, disclosed the use of a light activated pnpn light emitting diode (LA-LED) as a combination detector-discriminator. Devices of this type have been made which exhibit an S-type electrical negative resistance characteristic similar to a silicon Shockley diode. Thus, by using a suitable load impedance and bias voltage, these diodes can be made to have two stable states: (a) a high voltage, low current state, with little light output, and (b) a low voltage, high current state, with appreciable light output. It has been further shown that when the diode is in state "a", the incidence upon the diode of a relatively small optical signal will cause the diode to switch to state "b". In particular, for the illustrative diode described, a 3 microwatt incident light signal produced a 100 microwatt optical output signal along with a 1.4 volt electrical signal. The diode is reset to state "a" after a prescribed period of time by means of a passive RLC circuit which reduces the bias voltage across the diode.
The advantage of such a device resides in its simplicity. One application would be as the sole element in a tap and repeater circuit in an optical data bus. In this application optical input signal strengths would be of the order of 10 .mu.W. Outputs of 50 to 100 .mu.W would be adequate, since the optical fiber lengths in such an application would be relatively short. In addition to regenerating the optical signal, the electrical output would be adequate for driving TTL logic directly.
The pnpn detector-discriminator would also appear to be an attractive alternative to the optical repeater described by Goell. However, before such a device could be used in a long distance communication system, much greater sensitivity is required. This, in turn, depends upon how close the bias voltage can be set to achieve the optimum switching threshold as a function of signal strength. The difficulty, however, resides in the fact that with a switching detector it is not possible to obtain directly a measure of the input signal strength and, thereby, to develop a bias control signal for varying the threshold level as a function of the input signal strength.
It is, accordingly, the broad object of the present invention, to provide means for automatically controlling the threshold level of an optical detector-discriminator switch as a function of input signal intensity.